1. Field of Invention
The invention relates generally to hydraulic actuators and a method of using the same. The hydraulic actuator may be used in a system for braking vehicles, such as by actuating hydraulic brakes of trailers, and is well suited for use as a surge brake actuator for use with a hydraulic braking system for trailers.
2. Description of Related Art
For towed vehicles, such as trailers, it is and has been common, if not required, to provide a self-contained hydraulic braking system that operates independently of the braking system of the towing vehicle. A surge brake actuator permits the towed vehicle to actuate its own braking system when the towing vehicle decelerates.
Historically, a surge brake actuator generally included a coupler housing component mounted on the towed vehicle and attached to the towing vehicle and a hydraulic master cylinder component that actuates the braking system of the towed vehicle. These surge brake actuators operate such that when the towing vehicle decelerates, the forward momentum of the towed vehicle applies a force on the coupler housing. The hydraulic master cylinder utilizes the resultant force on the coupler housing component to generate fluid pressure to actuate the braking system of the towed vehicle. Specifically, the resultant force is translated into fluid pressure within the hydraulic master cylinder to activate the braking system of the towed vehicle.
The output pressure of the braking system was a function of the ability of the brake actuator to convert the forces provided by the towing vehicle into hydraulic pressure so as to actuate the braking system of the towed vehicle. This force/pressure ratio is an important component in the ability of an actuator to provide adequate braking pressure to the towed vehicle.
Historically, braked trailers in the United States have used electric drum brakes, partly due to their low price. The braking effectiveness of drum brakes in general and electric drum brakes in particular fades significantly as operating temperatures increase. Drum brakes also require substantial maintenance. Certain application, such as boat trailers, are inherently ill suited for electric brakes and therefore virtually all boat trailers use hydraulic brakes (both disc and drum) actuated by surge brake actuators.
Surge brake actuators were originally designed to actuate drum brakes, not disc brakes. They worked well in that they furnished (1) the necessary volume to take up the “hydraulic slack” out of the system (e.g., move the brake shoe to engage the brake drum) and to account for brake line/hose expansion and (2) provide the necessary pressure to properly actuate the brakes. Drum brakes do not require high pressures to operate because drum brakes are, to an extent, self-energizing. This is because the direction of rotation of the drum “pulls” the forward shoe towards the drum. This partially self-energizing design results in a situation where a small increase in brake line pressure can generate a substantial increase in brake torque.
On the other hand, disc brakes are not “self-energizing”. In fact disc brakes react rather linearly with the addition of more hydraulic pressure. For example, an increase of about 5% more hydraulic pressure may result in a 4% increase in braking force. Consequently disc brakes require more pressure for proper operation than do drum brakes.
However, today's surge brake manufacturers still offer basically the same actuator (same piston diameter and same stroke length) for both drum and disc brakes. This means that surge braked trailers with disc brakes are likely under-braked due to inadequate pressure being generated by the surge brake actuator. Therefore, when the towing vehicle has to slam on its brakes in an emergency situation, or for other reasons, the under-braked trailer will impair the braking performance of the combination of vehicles sufficiently to cause a collision between the towing vehicle and other nearby vehicles.
Two scenarios are common when the towing vehicle must perform an emergency stop. If the combination of vehicles (towing and towed vehicle) brake during a turn and the towed vehicle brakes do not have enough pressure to properly operate, the forward momentum of the towed vehicle will likely cause the combination of vehicles to jackknife. During the analogous situation while the combination is braking in a straight line, the forward momentum of the towed vehicle will add to the braking load of the towing vehicle, thereby lengthening the stopping distance of the combination.
These types of emergency stops happen thousands of times a year in the United States alone, causing significant human suffering and property damage. Therefore, a better surge brake actuator is needed.